Digital subscriber line (DSL) technologies allow existing twisted-pair telephone lines to communicate both analog telephone signals and digital data signals between, for example, network equipment at a central office and subscribers at remote locations. Typically, the telephone signal is communicated using a frequency band between approximately 300 kHz and approximately 3.5 kHz, and the data signal is communicated using a frequency band above approximately 25 kHz. At both the central office and a subscriber's remote location, a plain old telephone service (POTS) splitter may receive an input signal from a telephone line and separate telephone and data signals from the input signal using low-pass and high-pass filters, respectively. In addition, the POTS splitter may attenuates the spectral power from a DSL modem output to a telephone device input.
In general, a POTS splitter should satisfy two fundamental requirements to operate optimally in conjunction with a typical telephone device and a typical DSL modem. First, to avoid interfering with or degrading the quality of basic telephone services, the POTS splitter should have a characteristic impedance that suitably matches or approximates the impedance of the telephone line and the telephone device, which is typically approximately 900 Ω. Second, because the high-pass filter used to separate the data signals from the input signal typically has an approximately 50-60 nF capacitance to accommodate the relatively low input impedance of the DSL modem, the POTS splitter should suitably absorb this 50-60 nF capacitance.
As individuals, organizations, businesses, and governments continue to transition from a post-industrial economy to an information-based economy, their communication needs continue to grow. Consequently, telecommunications service providers must strive to communicate an increasing amount of information over existing twisted-pair telephone lines. For example, in many European markets, service providers use additional tones or other suitable signals, usually at an intermediate frequency of either approximately 12 kHz or approximately 16 kHz, to communicate tax, billing, or other information associated with telephone calls. Service providers in other markets may have similar needs. Unfortunately, telecommunications equipment manufacturers have been unable to provide a cost-effective POTS splitter to efficiently separate these additional signals of intermediate frequencies from input signals that are transmitted over telephone lines and contains low and high frequency components associated with telephone and data signals, respectively. More particularly, previous POTS splitters cannot adequately satisfy the two fundamental requirements described above while passing intermediate frequencies above approximately 6-8 kHz, such as those typically associated with tax, billing, or other information related to calls. As a result, previous POTS splitters are increasingly inadequate for many applications.